1. Field of the Invention
This invention relates generally to electro-acoustical drivers and loudspeakers employing electro-acoustical drivers, and more particularly, to improved configurations for compression drivers.
2. Related Art
An electro-acoustical transducer or driver is utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical ones. The basic designs and components of various types of drivers are well-known and therefore need not be described in detail. Briefly, a driver receives electrical signals and converts the electrical signals to acoustic signals. The driver typically includes mechanical, electromechanical, and magnetic elements to effect this conversion. For example, the electrical signals may be directed through a circular voice coil that is attached to diaphragm and the voice coil positioned in an air gap with a radially oriented permanent magnetic field provided by a permanent magnet and steel elements of a magnet assembly. Due to the Lorenz force affecting the conductor of current positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air space and, correspondingly, move the diaphragm to which the coil is attached. The voice coil may be attached to a flexible diaphragm that is suspended by one or more supporting elements (e.g., a surround, spider, or the like) such that at least a portion of the diaphragm is permitted to move. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level.
The driver at its output side may be coupled to an acoustic waveguide, which is a structure that encloses the volume of medium into which sound waves are first received from the driver. The waveguide may be designed to increase the efficiency of the transducer and control the directivity of the propagating sound waves. The waveguide typically includes one open end coupled to the driver, and another open end or mouth downstream from the driver-side end. Sound waves produced by the driver propagate through the waveguide and are dispersed from the mouth to a, listening area. The waveguide is often structured as a horn or other flared structure such that the interior defined by the waveguide expands or increases from the driver-side end to the mouth.
One type of electro-acoustical transducers or drivers is a compression driver. A compression driver produces sound waves in a high-pressure enclosed volume, or compression chamber, before radiating the sound waves to the typically much lower-pressure open-air environment. The compression chamber is open to a phasing plug, which is a device that works as a connector between the compression chamber and the horn. The area of the entrance to the phasing plug is typically made smaller than the area of the diaphragm to provide increased efficiency compared to other types of drivers, such as a direct-radiating loudspeaker. In a direct-radiating loudspeaker, the output mechanical impedance of the vibrating diaphragm is significantly higher than the radiation impedance that causes “generator” (diaphragm) and “load” (radiation impedance) mismatch. In a compression driver, the loading impedance (entrance to the phasing plug) is significantly higher than the open air radiation impedance. This produces much better matching between “generator” and “load” and increases the efficiency of the transducer. In general, compression drivers are considered to be superior to direct-radiating drivers for generating high sound-pressure levels.
As noted, a compression driver utilizes a compression chamber on the output side of the diaphragm to generate relatively higher-pressure sound energy prior to radiating the sound waves from the loudspeaker. Typically, the phasing plug is interposed between the diaphragm and the waveguide or horn portion of the loudspeaker, and is spaced from the diaphragm by a small distance (typically a fraction of a millimeter). Accordingly, the compression chamber is bounded on one side by the diaphragm and on the other side by the phasing plug. The phasing plug typically includes apertures (i.e., passages or channels) that extend between the compression chamber and the waveguide or horn portion of the loudspeaker to provide acoustic pathways from the compression chamber to the waveguide. The cross-sectional area of the apertures is small in comparison to the effective area of the diaphragm, thereby providing air compression and increased sound pressure in the compression chamber.
The compression driver, characterized by having a phasing plug and a compression chamber, may increase the efficiency with which the mechanical energy associated with the moving diaphragm is converted into acoustic energy. Decreasing the parasitic compliance of air in the compression chamber prevents undesired attenuation of high-frequency acoustic signals. Properly positioning of the apertures in the phasing plug and the lengths of the passages may permit delivery of sound energy in phase from all parts of the diaphragm, suppression or cancellation of high-frequency standing waves in the compression chamber, and reduction or elimination of undesired interfering cancellations in the propagating sound waves.
There exists a need for improved designs for compression drivers so as to more fully attain their advantages such as high-frequency efficiency, while ameliorating their disadvantages such as detrimental acoustical non-linear effects, irregularity of frequency response, and limited frequency range.